Basic intrusion detection systems that utilize active or passive detectors such as passive infrared sensors (PIR), microwave sensors, and ultrasonic sensors to detect the presence of an intruder in a protected zone are well known in the art. Radiation received from the protected zone is electronically processed and an alarm signal generated by the system when the received radiation exceeds a predetermined threshold. The utility of basic intrusion detection systems is limited by susceptibility to spurious alarms/alarm failure due to various Phenomena such as noise, variations in atmospheric conditions, random thermal activity, changes in placement/operability of protected zone equipment, and changes in actual versus design range of the system.
Combining detection signals is one means of improving alarm integrity, i.e., reducing spurious alarms. Dual element balanced detectors, as disclosed in U.S. Pat. Nos. 4,707,604, 4,514,631, 4,364,030, 4,343,987, and 3,839,640, provide common mode rejection of detection signals caused by randomly varying thermal activity. Balanced detectors utilize dual PIR sensors that are electrically connected in series opposition to produce opposite polarity signals. The combined signals from randomly varying thermal activity are self-cancelling over time.
Another means of reducing spurious alarms by combining detection signals is seen in verified intrusion detection systems wherein two separate detection signals are generated and subsequently combined to provide an alarm decision signal having a higher integrity. Signal verification may be used in single sensor technology systems such as PIR/PIR detection systems or in mixed sensor technology systems such as PIR/microwave and PIR/ultrasonic detection systems.
FIGS. 1A, 1B illustrate prior art verification systems for combining two independent analog detection signals to generate a decision signal having higher integrity. Signals A and B are analog signals typically generated by PIR, microwave, and/or ultrasonic sensors and processing subsystems. In general, analog detection signals exhibit an increase in amplitude when an intruder is present within the protected zone. When both analog detection signals A, B are greater than a unity threshold magnitude, the decision signal causes an alarm to be triggered, i.e., AND verification. Discussed hereinbelow are two known methods to accomplish AND verification.
FIG. 1A illustrates one exemplary AND verification subsystem 100 for processing analog detection signals A, B generated by sensor and electronic processing subsystems (not shown) of an intrusion detection system. Each analog detection signal A, B is fed to a pair of comparators 102, 104 for comparison to predetermined detection thresholds, V.sub.H, V.sub.L, respectively. Each pair of comparators 102, 104 is coupled to an OR gate 106 such that a detection signal exceeding the detection threshold causes the corresponding OR gate 106 to generate an output signal. The OR gates 106 are coupled to an AND gate 108 that generates a decision signal 110 to trigger an alarm. Both OR gates 106 must generate an output signal before the AND gate 108 will generate the decision signal 110.
Another exemplary embodiment of an AND verification subsystem 200 is illustrated in FIG. 1B, and includes pairs of comparators 202, 204 biased for predetermined detection thresholds V.sub.H, V.sub.L, two AND gates 206, and one OR gate 208. In this embodiment the outputs of the high threshold comparators 202, 202 and the low threshold comparators 204, 204 are coupled to corresponding AND gates 206 as illustrated. When both analog detection signals A, B simultaneously exceed a predetermined threshold, either V.sub.high or V.sub.low, the corresponding AND gate generates an output signal. An output signal from either AND gate 206 causes the OR gate 208 to generate a decision signal 210 that triggers an alarm.
One limitation of these types of systems is that both input analog detection signals must be over a predetermined unity threshold at the same time. This operating condition usually results in decreased system sensitivity, or even no detection at all. There are several reasons for such decreased sensitivity.
In PIR/PIR verified detection systems the upper sensing elements are referenced at the upper parts of the human body which generally transmits significantly more infrared radiation than the lower parts of the body. This occurs since the head and hands are relatively hot and unclothed while the lower body portions are usually clothed and as a result generate less infrared radiation contrast. To compensate for these differences in target contrast, the detection system normally requires enhanced sensitivity which may be achieved by increasing the gain of the amplifiers or by reducing the detection threshold levels. Furthermore, such detection systems generally have mounting constraints as the A and B sensor elements will receive less energy if the mounting height is not optimum for the target.
In PIR/microwave and PIR/ultrasonic detection systems the preferred direction for the optimal detection zone is under 45 degrees as referenced to the detector. For operation in this detection zone, both sensing technologies generate comparable signal levels. In addition, for these detection systems it may be necessary to increase the sensitivity of both sensor subsystems to avoid stringent walk directional limitations.